1. Field of the Invention
The present invention relates to an image display apparatus and, more particularly, to an image display apparatus, for example, a head-mounted image display apparatus, which uses a compact optical system and yet has a large exit pupil diameter.
2. Background of Related Art
Helmet- and goggle-type head-mounted image display apparatuses have heretofore been developed for the purpose of enabling the user to enjoy virtual reality or wide-screen images personally.
Conventional head-mounted image display apparatuses are generally arranged such that an image displayed on a two-dimensional display device, e.g., a liquid crystal display device, is projected on the user's retina as an enlarged image by using an ocular optical system (e.g., Japanese Patent Application Laid-Open (KOKAI) No. 4-170512). However, if the ocular optical system has a large numerical aperture, aberrations are likely to occur, and it is necessary in order to prevent occurrence of aberrations to use a complicated arrangement and a large-sized lens. To solve this problem, a head-mounted image display apparatus in which the numerical aperture is limited at the illumination system side has been proposed (Japanese Patent Application Laid-Open (KOKAI) No. 3-214872) of which U.S. Pat. No. 5,371,556 is a family member.
However, when the numerical aperture of the ocular lens is small, it is likely that light rays will be eclipsed by the pupil of the user's eye. This problem will be explained below with reference to FIGS. 54(a) and 54(b) in the accompanying drawings. FIG. 54(a) shows an essential part of a conventional head-mounted image display apparatus. An illumination system 1 illuminates a liquid crystal display device 2 from behind it, thereby allowing an image displayed on the liquid crystal display device 2 to be projected on the retina 5 of a user's eyeball 4 as an enlarged image by a convex lens 3 constituting an ocular optical system. To minimize aberrations produced by the convex lens 3, the illumination system 1 illuminates the liquid crystal display device 2 with a small numerical aperture. Accordingly, the diameter of the exit pupil 6 of this head-mounted image display apparatus is small, as shown in FIG. 54(a). If the user wears the head-mounted image display apparatus such that the exit pupil 6 is precisely coincident with the position of the user's pupil 7, light rays emanating from central and peripheral images P.sub.1 and P.sub.2 on the liquid crystal display device 2 pass through the pupil 7, as shown in FIG. 54(a), as long as the user gazes at the center of the displayed image. Therefore, the user can observe the whole image displayed on the liquid crystal display device 2, from the center to the periphery of the displayed image. It should be noted that reference symbols P.sub.1 ' and P.sub.2 ' denote the images P.sub.1 and P.sub.2 projected on the retina 5.
However, when the exit pupil 6 and the position of the user's pupil 7 are not precisely coincident with each other, the light rays are eclipsed by the pupil 7, resulting in a darkened image. When the user looks at the peripheral image P.sub.2 ', as shown in FIG. 54(b), the eyeball 4 rotates about the point of rolling, which lies in the vicinity of the center of the eyeball 4. Therefore, the position of the pupil 7 shifts, and it becomes even more difficult for the light rays emanating from the central and peripheral images P.sub.1 and P.sub.2 to pass through the pupil 7. In an extreme case, it becomes impossible to observe the displayed image.
As a method of solving the above-described problems, there is a method in which, as shown in FIG. 55, the pupil diameter is enlarged by disposing two diffraction gratings 11 and 12 in parallel to each other (Japanese Patent Application No. 6-41166). In the head-mounted image display apparatus, shown in FIG. 55, an image displayed on a liquid crystal display device 2 is projected in a user's eyeball as an enlarged image by a convex lens 3 constituting an ocular optical system. The liquid crystal display device 2 is illuminated by an illumination system 1 disposed behind it to project the image displayed thereon. In the conventional apparatus, a pair of diffraction gratings 11 and 12, which have the same grating interval, are disposed in parallel to each other between the convex lens 3 of the ocular optical system and the exit pupil 6 thereof such that the grating directions of the diffraction gratings 11 and 12 coincide with each other. By virtue of the above-described arrangement of the pair of diffraction gratings 11 and 12, a parallel beam of light passed through the convex lens 3 first enters the diffraction grating 11 and separates into zeroth-order diffracted light, +1st-order diffracted light, and -1st-order diffracted light. These diffracted beams of light then enter the diffraction grating 12, which has the same grating interval as that of the diffraction grating 11. Thus, these beams of light are diffracted again, and a part of the light becomes a parallel beam. As a result, the beam diameter a of the light before entering the diffraction grating 11 enlarges to a beam diameter b after the light has emanated from the diffraction grating 12. Thus, the effective pupil diameter enlarges.
Since the above-described conventional method makes it possible to enlarge the pupil diameter in between the ocular optical system and the user's eye, the ocular optical system can be reduced in size. However, the diffraction angle of the diffraction gratings 11 and 12 depends largely on the wavelength, and large chromatic aberration occurs at the position of the exit pupil 6. Consequently, as the eye is moved, the projected image undesirably looks changeable in color.